Our research interests are centered on understanding the physics behind complex material behavior and their technological applications, especially in relation to cooperative structural change ranging from solid-state structural phase transitions, liquid-glass transitions to protein folding and other cooperative phenomena.
For example, near a phase transition point, systems exhibit large-amplitude fluctuations and behave in a highly correlated manner. In the case of proteins, far-from-equilibrium structures formed upon denaturation are able to undergo a sequence of large-amplitude motions necessary to reach the folded, active state. In addition, systems readily show nonlinear responses to external fields in this critical region. How is this cooperative behavior generated from random fluctuations? How will materials respond dynamically, in the linear and nonlinear regimes, to complex external stimuli, and how is this related to the formation of order? How can the materials behavior be used in a technologically useful way? To answer these questions, we use advanced spectroscopic techniques employing femtosecond lasers to excite electrons, phonons, spins or other degrees of freedom in a controlled manner, and to observe how the excitations evolve over a wide range of timescales, with special attention paid to where the energy flows and what are the coupled degrees of freedom. THz spectroscopy is particularly useful to probe the low frequency excitations in condensed matter systems. Typical examples of these excitation include soft phonons near a structural phase transition point, Boson peak in disordered materials, conformation dependent low frequency absorption in biomolecules, etc. Our research is focused on understanding the underlying mechanisms that dictate the behavior of complex materials, but these goals also include the development of novel spectroscopic techniques for control of materials properties, such as formation of domains, nucleation, and other types of order using short laser pulses, which should open new possibilities in electronics, optical storage, and optoelectronics.